This tool is useful for quickly inspecting how Firefox renders your web pages as it visually indicates how much screen area is being painted after the page layout is computed.The source code for the add-on is also posted.

I suspect that there are other programmers out there currently lurking around the Mozilla community, intimidated by the scale of the source tree, and wondering where to start hacking on Firefox. I highly recommend starting with the Gecko Overview started by L.David Baron, Mozilla Principal Software Engineer, to help beginners understand the browser engine. Thanks, David!